Automatic tone control



Oct. 21, 1952 w J GRUEN 2,615,090

AUTOMATIC TONE CONTROL Filed April 1, 1948 Fig.2.

.5 22 D 2 UJ ,2

0 5'0 :00 npoo 4 10,000

FREQUENCY CYCLES PER secbun lnventori His Attorney.

Patented Got. 21, 1952 AU'roMATIoToNE CONTROL Wolf J. Gruen, Syracuse,N. Y., assignor to General Electric York 1 Company, a corporation of NewI Application April 1,1948,SerialNo.18,339

-- This invention relates to automatic tone control circuits, and moreparticularly to such circuits useful for controlling the frequencyresponse of audio frequency amplifiers, audio reproducing systems, andthe like. I

It is an object of this invention to provide an automatic tone controlcircuit which maybe utilized for controlling the frequency response ofan audio frequency amplifier, and the like, the tone control circuitacting to broaden or narrowthe frequency response of the amplifier andbeing actuated by a control potential which is inturn dependent upon theenergy content of signals amplified by the amplifier and lying solelywithin apredetermined frequency range.

In the attainment of the foregoing objects thereis provided an audioamplifier havinga variable frequency response, this response beingvcontrolled by the proposed tone control circuit.

Frequency selective filter means and rectifying.

means are included in the control circuit, and from these means acontrol potential is'derived, the value of the control potential beingdependent upon signals amplified by the amplifier, lying, for examplewithin the frequency range of300- 1500 C.- P. S. As a major portionof the energy content of musical programs lies within this frequencyrange, the frequency response of the amplifier is increased during suchprograms, and high quality translation is thereby obtained. However,during the-intervals between the musical passages, the tone controlcauses the-frequem' cy response of the amplifier to decrease so thatthe. limits thereof are substantially 300- 1500 C. P. S.,' anddisturbing scratch, rumble and other interference, beingoutside thisfrequency range and normally masked by the music, are effective lyattenuated. I

It is proposed, for example, to utilize the abovedescribed system inrecord reproducing devices,

as the disturbing noise frequencies from the record generally falloutside the above-mentioned predetermined frequency range. These noisesare therefore suppressed during thesilent passages due to the tonecontrol circuit, limiting the bandpass 'characteristic of the amplifier,but during the presence of musical passages when the frequency responseof the amplifier is increased by the tone control circuit, thesedisturbances are:

4 Claims. (01.178-44) erence to the accompanying drawing in which Fig. 1is the circuit diagram of an audio amplifier incorporating the proposedautomatic tone control circuit, and Fig.2 shows various response curvesof the system. Referring now to Fig. 1, audio signals from anyappropriate source are applied to terminals I, these terminals beingconnected across series connected grid-leak resistors 2 and 3 which arein turn connected between the control electrode of discharge device 4and ground. The cathode of device 4 is connected directly to ground, andthe anode of this device is connected to a unidirectional potentialsource B+ through load resistor 5. Output from device t is derivedacross resistor 5 and applied to control electrode 8' of dischargedevice 8 through capacitor 6. The con-' trolelectrcde 8 is connected toground through resistor 1, and the cathode of device 8 is biased bymeans of resistor 9 bypassed foraudio frequencies bycapacitor ID.Suppressor electrode 8" is connected to the cathode of device 8, and"screen 8" is connected directly to the unidirectional potential source13+. The anode of device 8 is connected to unidirectional potentialsource B+ through the primary winding I I of audio output" transformerII, the secondary Winding ll" of this transformer being connected to asignal translating device l2.

' The output potential across the primary winding H of audio outputtransformer II, or a por tion thereof, is applied to the controlelectrode of discharge device l9 through a capacitor 13, and shuntconnected capacitor I5 and resistor l4, discharge device l9 being of thevariable mu type. Thecontrol electrode of device [9 is connected toground through series connected resistor I 6 and capacitor H3. Thecathode of device I9 is directly connected to ground, and the anode ofthis device is connected to the unidirectional potential source B+through resistor 20. The alternating current component of the potentialappearing across the resistor 20 is applied to the input of device 4through series connected resistor 2| and capacitor 22, and a connectionto the junction point of series resistors 2 and 3.

to being applied tothe control electrode of device I9, is also appliedto a resonant circuit through capacitor [3 and resistor 23, the resonantcircuit comprising an inductance 24 and capacitor 25 connected inparallel. The low potential end of resonant circuit 24, 25 is connectedto ground. The cathode of device 28 is connected to the junction pointof resistor 23 and resonant circuit 24, 25, and the anode of device 28is coupled to ground through a 'shunt network pomprising resistor andcapacitor 21. Output'from device 28 is utilized to bias device 19, andis applied to the control electrode of the latter device throughresistor I! connected to the commonjunction of resistor l6 and capacitorl8.

The components of the circuit, indicated as 2-l2 in Fig. 1, form aconventional audio :amplifier. The amplifier may include a usual manualtone control, not shown, in orderto permit "an adjustment of the overallfrequency response of the system, in accordance with the programtolbereproduced. It is assumed, for example, that the frequency responsecharacteristic of 'the amplifier aloneis-fiat from 50 to 10,000 C. P.=S., ,asrshown by curve AiniFig. 2. I

The circuitof Fig. 1, when the-diode $28, and

associated circuit elements areexcluded, comprises avnegativefeed-backamplifier. By-insertingthe band: elimination-filter [4,: I'5,=L6,: l.8:in the feed-back path, the frequency rresponse .of theamplifier is narrowed .as shown by 1 curve {B ,-in Fig. 2. Theabove-mentioned filter preferably is a high impedance -network ,so,that. loading of the primary winding ll of transformer .11 is avoided..The. amountof attenuation thatimay'rbe obtained isdetermined by therelative .-sizes ..of resistors .44 andHi. vThe low frequency cuteofidependston the relative valueszofiresistor [Er and capacitor I8,'and thehigh-,frequencyecut-eoff .is determined by. the. relative. sizes .ofresistor I iand capacitor l5. The amount .of feed-.back'may beadjusted-by the appropriate choiceof resistors '2, 3,' and 2 I.

"Considering nowathe operation of .the, complete circuit, including thediode 28 and,its associated components, any audio potential appearing:across the-resonant circuit-2.4,2S-isuectified by diodeJB,

and a negative unidirectional ipotenti-al ,isiobtainedat the anodethereof. ,The resonantcircuit 24, ZS-together with resistorl2-3 form a:band pass filter, the frequency characteristic v.oi' which is shown bycurve C- of Fig.-l2. ,Assumingsquare-law detection, the negativepotentialv at the anodetof devices 28 is proportional to :the energycontent within the .pass-band-of:fi1ter r23, r24, 15. :By applying thispotential to the control electrode of the variablemusdevice, t9, .theamount. of feed-- backiis reduced, andtheirequency responseof theamplifier is widenedwith increased negative control electrode bias ondevice =l9, this .being indicated by the dotted .curved D, 1Df, .of-Fig. ,2. Frequencies outside the passband of fil ter- 23, 24, 25, suchas record noise, :do not ..develop any appreciable potential acrossresonant.circuit 14, 25, ,and therefore ha-veno control effect .on .thefrequency response of theamplifier.

.To increase the.fide1ity of the amplifierpdevice I9 is biased towardscut-off so that aminimum amount of distortion is introduced .by.thecontrol device, and inthe signal translated by thesystem.

It is. also noted that by adjusting the maximum feed-back in the centerofthe predetermined frequency'band of the amplifier to anappropriateamount, a desirable volume expansion maybe obtained.

This invention therefore provides an amplifier system having a controlcircuit incorporated therein, whereby, signalslying within apredetermined pass-band are effective to change the frequency responseof the amplifier, and signals lying outside this pass-band have littleor no effect on the amplifier frequency characteristics.

While a certain specific embodiment of the invention has been shown anddescribed, it will of course he understood that zvariousmodifications-:maybe made without departing therefrom. The appendedclaims are intended to cover any 'such modifications as fall within thetrue spirit :.and' scop,e of .the invention.

What'I 'claim as new and desire to secure by Letters Patent of theUnitedStates is:

FlHAn :a'udio amplifying system having an input circuit and an outputcircuit, means for impressingsignals on said input circuit havingfrequencies extending over a predetermined range of :audio frequencies,means for deriving amplified output signals from said output circuit,means for a varying .the ifrequencyzresponse of --=said-amplif.yingasystemain =response'ltozsignals amplified :therebyand lying .withinzaifirsttpredeterminednarrow audio'frequencyzband,tsaidrlastmentionedmeans comprising.anr'inversei'feedback circuit coupling :said output:circuit in :said input circuit, :frequency :selective :control imeanszincluded ,in said feedback :circuit, said control means .being arranged'to :supply aputpntxsignals withinasaid .band zdegenerativelymorsaidiinput circuit,:additionalzfrequency;selectivezcircuitameansand-:rcctifyingimeans:coupledrtoxsaid 'outpu-tscircuit if or deriving eaunidirectional :control :poten tial iin response to .output signalsflying within band, the :magnitude i'Of :said :control potential beingdependentiupon' the energyscontent offsaid lastementionerl signals,rand'imeans .for impressingssa-id control potential: onssaid controlmeans for controlling thefeedback and'athe frequency I baclco'f sai'dsystem,:said:dischargerdeviceiincluding-a: control electrode,:frequencyxselective :filter means and rectifying :means coupled .tosaid output circuitzfor derivingrza'nnidirectional:control' potential;in 'response 'to output :signals llying within-1a secondzfreguency bandzincluded withizra portion of said rfirst predetermined 'frequency band,rthe 'magnitude of .said :control :potential beingdependent upon theenergy contenti'ofisaid last-mentioned signais,'and means for-impressingsaid control ,potential on :said ECOILtTOl electrode for controlling thefeedbackcvand the frequencyresponse characteristicsof said system.

.3" An .audio amplifying system having afrequency response dependentupon signals amplified therebyand. lying within a certain predeterminednarrowvaudio frequency band, .saidamplifying system comprising. an inputcircuit and an output circuit, means for impressing audio frequencysignals on said input circuit and means for deriving amplified signalsfrom said output circuit, a feedback circuit coupling said outputcircuit to said input circuit, frequency selective filter means includedin said feedback circuit for limiting the frequency range of signals fedback to said input circuit to another predetermined audio frequencyband, means comprising an electron discharge device in said feedbackcircuit for controlling the amplitude of said feedback signals, saiddevice having a control electrode, further frequency selective filtermeans and rectifying means coupled to said output circuit for deriving anegative unidirectional potential therefrom in response to signals lyingwithin said predetermined frequency band, the magnitude of said controlpotential being dependent upon the energy content of said last-mentionedsignals, and means for impressing said control potential on said controlelectrode for controlling the feedback and the frequency responsecharacteristics of said system.

4. In an audio amplifying system having a uniform response over apredetermined relatively wide frequency band, an input circuit and anoutput circuit, means for impressing signals on said input circuit andmeans for deriving amplified output signals from said output circuit,

means for reducing the uniform frequency response band of saidamplifying system comprising, a frequency selective feed-back circuitcoupling said output circuit to said input circuit, control meansincluded in said feed-back circuit, and means for controlling saidfrequency band reducing means comprising frequency selective circuitmeans and rectifying means coupled to said output circuit for deriving aunidirectional control potential in response to output signals lyingwithin a relatively narrow frequency band Within a portion of saidreduced frequency band, the magnitude of said control potential beingdependent upon the energy content of said last mentioned signals, andmeans for impressing said control potential on said control means forcontrolling the frequency band reducing effect of said feed-backcircuit.

WOLF J. GRUEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number I Name Date 2,152,613 1 Wheeler Mar. 28,1939 2,179,956 Roberts Nov. 14, 1939 2,283,241 Van Cott May 19, 19422,323,634 Van Slooten July 6, 1943

